In the related art, as a means of removing a large calculus that has grown in the bile duct from the bile duct, there is a known calculus-crushing treatment tool with which a calculus is crushed by using a basket formed of wires (for example, see Patent Literature 1).
With this treatment tool, the basket in which the calculus is captured is pulled into a sheath from a distal-end opening in the sheath, the basket is contracted, and thus, the calculus is crushed by squeezing the calculus by contracting the basket.
In the calculus-crushing treatment tool, because the compression force exerted on the sheath when crushing the calculus is high, in Patent Literature 1, a coil sheath having excellent compression resistance is used. Also, a resin inner cylinder is disposed on the inner side of the coil sheath in order to reduce the friction due to irregularities on the inner surface of the coil sheath, and thus, a smooth motion of the basket is ensured.
In the case in which the lateral cross-section of the strand constituting the coil sheath has a circular shape, there is a risk of portions of the strand buckling by shifting in radial directions of the coil sheath with respect to each other when the compression force is increased. If the strand diameter and the coil-sheath diameter are increased in order to avoid this risk, the flexural rigidities thereof are increased, which makes the work involved in inserting the coil sheath into the bile duct from the duodenal papilla difficult.
As a buckle-resistant coil sheath, there is a known coil sheath in which the strand has a lateral cross-section having depressions and protrusions, and in which adjacent portions of the strand are brought into contact with each other by means of surface contact between the depressions and the protrusions (for example, see Patent Literature 2).
With this coil sheath, as compared with the coil sheath having a circular lateral cross-section, buckling is suppressed by means of the friction due to surface contact between the relevant portions of the strand. Therefore, it is possible to maintain the compression resistance thereof even if the width of the coil sheath is reduced in the radial direction.